This invention relates to a redundant automation system comprising a set of redundant PLCs composed of a pair of PLCs each equipped with two communication couplers, the first coupler in the first PLC being connected to the first coupler in the second PLC through a first Ethernet network, the second coupler in the first PLC being connected to the second coupler in the second PLC through a second Ethernet network connecting them to at least one remote equipment.
In order to guarantee operating availability of some processes, it is necessary to use a redundant automation system composed of two conventional PLCs. With reference to FIG. 1, each programmable logic controller AP-A or AP-B comprises two communication couplers CC1-A or CC1-B and CC2-A or CC2-B. The two couplers CC1-A and CC1-B are connected to each other through an ETHERNET type network BE1 to ensure that the processing carried out on each is consistent. The other two couplers CC2-A and CC2-B are connected to the other ETHERNET type network BE2 and enable the redundant automation system to communicate with third party equipment. These PLCs AP-A and AP-B each receive signals from sensors C on the input channels of couplers CA-A and CA-B or send control signals to the control devices in the automated installation.
The purpose of this invention is to provide a redundant automation system operating under the TCP/IP protocol. The redundant system is accessed through a unique IP address, the system being seen like a singular PLC. No specific modification has to be made to applications running on third party equipment. When switching takes place between PLCs within the redundant system following an operating problem, it occurs very quickly and therefore loss of communication with third party equipment is minimized, or is even imperceptible.
The automation system according to the invention is characterized by the fact that each of the PLCs has a hardware address and an IP address, all redundant PLCs being accessible from outside through a single IP address assigned to a first xe2x80x9cnormalxe2x80x9d PLC, while a switching mechanism assigns the address related to normal operation to the second PLC, the standby operation IP address being assigned to the first PLC.
According to one characteristic, each remote equipment that dialogs with the set of PLCs comprises a pair consisting of the IP address and the hardware address, in a cache memory for each PLC.
According to one characteristic, when switching takes place, existing connections with the pair of addresses are closed and connections are re-opened with the new pair of addresses.
According to another characteristic, each coupler periodically sends a xe2x80x9cfree ARPxe2x80x9d request on the network, that enables any remote equipment connected to the network to take account of any change in the IP address/hardware address pair due to switching of the system.